A power transmission tower is a structure adapted to support conductors and lightning wires of high voltage overhead transmission lines. The existing high-voltage transmission tower is made from a steel material, and consisted of three parts: tower head, tower body and tower leg. For example, the Chinese Patent Application No. 201010297983.1 has disclosed a high voltage structure of typical model with a shape like a Chinese character ‘gan’ (). A cross arm made of conductive metal material and extended form a tower body that is electrically grounded can be used to suspend the transmission conductors to support the weight of the conductors. The transmission conductor needs to keep a certain electrical safe distance from the ground point to avoid flashover accident caused by ground discharge, so the metal cross arm should have a sufficient length and a long insulator string is added on the free end of the cross arm, to make the charged transmission conductor spaced from the grounded metal cross arm to ensure the insulation distance.
Because the existing high-voltage transmission tower adopts an all-steel structure, it has defects of heavy weight, easy to be rusted or crack and poor performance at a low temperature, and it also has difficulties in construction, transportation, operation and maintenance. Furthermore, considering that the wind load may make the suspension insulator string be tilted at a wind yaw angle to cause a decrease in the air gap between the transmission line and the tower body of the power transmission tower during normal operation, in order to secure the air gap between the transmission line (charged body) and the tower body (grounded body) from being broken down, the impact of the wind yaw angle should be considered when the design of the cross arm that the length of the cross arm should be increased accordingly, which must lead to increase the width of the entire transmission line corridor and the floor space of the entire transmission line.
In view of the above defects in the steel tower, the applicant of the present invention has presented a Chinese Patent Application No. 201010115171.0 related to a composite insulating tower for power grid transmission line. In the composite insulating tower disclosed by the application, because a cross arm made of a composite material is served as a support for transmission line, the length of the insulator string can be reduced or the suspension insulator string can be avoided to be used, so as to reduce the length of the cross arm, the cost of the tower, the weight of the tower and the floor space occupied by the power transmission tower. However, the cross arm of the composite tower only consists of one insulation rod, which is relatively thin and cannot satisfy the support for the high voltage transmission line.